Welcome, fellow PLC professionals! As technicians in the field, we all know the importance of understanding the inner workings of the devices we work with. Today, we will be delving into the scanning working principle of PLCs, a fundamental concept that serves as the backbone of their functionality. So, let's dive in and explore this topic further.
Firstly, let's define what we mean by scanning in relation to PLCs. In simple terms, scanning is the process of continuously checking and updating the status of inputs and outputs to and from the PLC. This is crucial for the PLC to make decisions and execute the desired tasks as per the program instructions.
The scanning process can be broken down into three main stages: input scan, program scan, and output scan. Let's take a closer look at each of these stages and their role in the scanning process.
Input Scan: As the name suggests, this stage involves the PLC scanning all of its input devices to detect any changes in their status. Inputs can be in the form of switches, sensors, or any other device that is connected to the PLC. The input scan takes place continuously at a high speed, typically in milliseconds, to ensure that any changes are detected promptly.
Program Scan: The program scan is where the magic happens. This stage involves the PLC reading the program instructions, which are typically written in ladder logic or other programming languages. The PLC then executes these instructions in a sequential manner, one after the other, to determine the desired output based on the input status detected in the previous stage.
Output Scan: Once the program scan is complete, the PLC moves on to the output scan stage. Here, it updates the status of the output devices, such as motors, valves, or lights, depending on the program instructions. This is a crucial stage as it allows the PLC to control the desired output action based on the input status and program logic.
Now that we have a basic understanding of the three stages of PLC scanning, let's take a closer look at the actual process and how it works.
The PLC scanning process begins with a trigger, which can be a manual input or a timer interrupt. This signal initiates the input scan stage, and the PLC starts reading the status of all its input devices. Once this stage is complete, the PLC moves on to the program scan stage, where it executes the program instructions and determines the output status. Finally, the PLC enters the output scan stage, where it updates the status of all its output devices, and the cycle repeats.
One important aspect of PLC scanning is the concept of scan time. This refers to the time it takes for the PLC to complete one full scan of all its inputs, program instructions, and outputs. The scan time varies based on the complexity of the program, the number of inputs and outputs, and the processing speed of the PLC. As technicians, it is essential to keep an eye on the scan time and optimize it whenever possible to ensure efficient PLC performance.
Another critical factor to consider in PLC scanning is the scan cycle. This refers to the interval between two consecutive scans of the PLC. A shorter scan cycle allows for faster response time but may also put a strain on the PLC's processing speed. On the other hand, a longer scan cycle may cause a delay in response time but puts less strain on the PLC. Finding the right balance is crucial, and it may vary depending on the application.
In conclusion, the PLC scanning working principle is the foundation of their functionality. Without proper scanning, the PLC would not be able to make decisions and execute tasks based on the program instructions. As technicians, it is our responsibility to have a thorough understanding of this concept and ensure that proper scanning is taking place in our PLC applications. I hope this article has provided you with a better understanding of the scanning process. Happy scanning, everyone!